This invention relates to a process for the treatment of crude oils and, more particularly, to the hydroconversion of heavy hydrocarbons in the presence of additives and catalysts to provide useable products and further prepare feedstock for further refining.
As the reserves of conventional crude oils decline, heavy oils must be upgraded to meet world demands. In heavy oil upgrading, heavier materials are converted to lighter fractions and most of the sulfur, nitrogen and metals must be removed. Heavy oils include materials such as petroleum crude oil, atmospheric tower bottoms products, vacuum tower bottoms products, heavy cycle oils, shale oils, coal derived liquids, crude oil residuum, topped crude oils and the heavy bituminous oils extracted from oil sands. These heavy hydrocarbon feedstocks may be characterized by low reactivity in visbreaking, high coking tendency, poor susceptibility to hydrocracking and difficulties in distillation. Most residual oil feedstocks which are to be upgraded contain some level of asphaltenes which are typically understood to be heptane insoluble compounds as determined by ASTM D3279 or ASTM D6560. Asphaltenes are high molecular weight compounds containing heteroatoms which impart polarity.
Heavy oils must be upgraded in a primary upgrading unit before it can be further processed into useable products. Primary upgrading units known in the art include, but are not restricted to, coking processes, such as delayed or fluidized coking, and hydrogen addition processes such as ebullated bed or slurry hydrocracking (SHC). As an example, the yield of liquid products, at room temperature, from the coking of some Canadian bitumens is typically about 55 to 60 wt-% with substantial amounts of coke as by-product. On similar feeds, ebullated bed hydrocracking typically produces liquid yields of 50 to 55 wt-%. U.S. Pat. No. 5,755,955 describes an SHC process which has been found to provide liquid yields of 75 to 80 wt-% with much reduced coke formation through the use of additives.
In SHC, a three-phase mixture of heavy liquid oil feed cracks in the presence of gaseous hydrogen over solid catalyst to produce lighter products under pressure at an elevated temperature. Iron sulfate has been disclosed as an SHC catalyst, for example, in U.S. Pat. No. 5,755,955.
Some reported SHC catalysts employ molybdenum as the active species. Molybdenum has been shown to have a stronger hydrogenation function compared to iron. However, molybdenum is more expensive than iron. Moreover, even at the very low concentrations in parts per million required for sufficient conversion, molybdenum catalysts may need to be recoverable to be cost effective. Such low concentrations of molybdenum are difficult to reclaim as they are highly diluted in the product streams.
During an SHC reaction, it is important to minimize coking. Asphaltenes present as a byproduct from the SHC reaction product can, if not managed properly, self-associate, or flocculate to form larger molecules, generate a mesophase and precipitate out of solution to form coke. Mesophase formation is a critical reaction constraint in SHC reactions.